Organopolysiloxanes having an atomic ratio of oxygen to silicon of 1.5 are generally called polysilsesquioxanes. These resins are generally utilized as abrasion-resistant coatings particularly on plastic lenses and glazing material. Polysilsesquioxanes have a ladder-like structure represented by the formula ##STR1## wherein R is the same or different and is lower alkyl, halolower alkyl, phenyl, substituted phenyl, phenethyl and methylphenethyl, with at least one of the groups represented by R being lower alkyl, and n is an integer. Coating compositions containing such resins are resistant to high temperatures and are hard. The hardness is due to a high degree of cross-linking in the resin.
Disadvantages of the coating compositions containing polysilsesquioxane resins are discussed in U.S. Pat. No. 4,746,693. In U.S. Pat. No. 4,746,693 which is incorporated herein by reference in its entirety, the disadvantages of the prior art coating compositions containing polysilsesquioxane resins are overcome by combining one or more silanes represented by the formula R--Si(OR).sub.3 wherein the groups represented by R are the same or different and are lower alkyl containing 1 to 6 carbon atoms, halolower alkyl containing 1 to 6 carbon atoms, phenyl or substituted phenyl, with the proviso that at least one group represented by R is lower alkyl; an effective amount of a nonvolatile, nonionic catalyst; a nonionic fluorochemical surfactant; and an amount of water representing from about 10 to about 95 mole percent of the moles of silanes present; agitating the mixture to form an emulsion; and maintaining the emulsion for a time sufficient to partially hydrolyze and condense the silanes to the extent permitted by the amount of water present. In U.S. Pat. No. 4,746,693, alkyl silanes are partially condensed to form a stable mixture predominately of dimers and tetramers by emulsion hydrolysis in the presence of a nonvolatile, neutral tin catalyst and a nonionic fluorochemical surfactant. The resulting stable preparation will completely cure upon exposure to atmospheric moisture to form a polyalkylsilsesquioxane resin coating. A novel resin, polymethylisobutylsilsesquioxane, is particularly advantageous in U.S. Pat. No. 4,746,693 for coating applications on space hardware.
Although the polyalkylsilsesquioxane coating compositions of U.S. Pat. No 4,746,693 provide excellent coatings for substrates, for example, for satellites and other space hardware which must be in low earth orbit where they will be subjected to high temperatures, intense UV radiation and atomic oxygen, it is always desirable to improve the properties of such coating compositions. For example, it is desirable to improve the oxidation resistance, the high temperature mass loss properties, adhesion and the curing characteristics. It is also desirable to reduce the number of ingredients in coating compositions and to simplify compounding and processing in the preparation of such coating compositions.